How can I attack resource tiles when it is occupied by an enemy?

When the enemy takes a plot there will be a 5 minute cool down when you cannot attack it. After that you should be able to occupy although it will possibly take you more than one hit to take it over. You will need to have the right level of immunity to occupy it of course.

You need to choose 'gather', not 'occupy' - this will make your troops attack the troops gathering there. Remember to recall them immediately afterwards!